1. Field of the Invention
The present invention relates to a passive seat belt apparatus, and more particularly to an apparatus which has means for protecting a relay circuit for controlling a direction of rotation of a seat belt drive motor.
2. Related Background Art
FIG. 1 shows a drive circuit for a motor used in an apparatus of this type.
Let us assume that when a door is opened, the motor is forwardly rotated to move a slider connected to one end of a seat belt toward a front end of a body (that is, a relay RL1 is energized), and when the door is closed, the motor is reversely rotated to move the slider backward (that is, a relay RL2 is energized). A voltage waveform of the relay circuit and a current waveform of the motor are shown in FIG. 2. When the relays RL1 and RL2 are energized and deenergized, large rush current spikes and cutoff current flow in the motor, respectively.
When the door is closed while the slider is moved forward, the rush current by the relay RL2 is added to the cutoff current by the relay RL1 as the motor is switched between forward rotation and backward rotation. As a result, a very large spike current flows. This is clear from FIG. 3 which is similar to FIG. 2. The same phenomenon takes place when the door is opened while the slider is moved backward. Since the relay contact is switched at that time, the relay contact is adversely affected. Accordingly, a relay having a large rated power (or contact capacity) is used in the prior art apparatus.